The variation in the Earth's gravity field causes a significant error in autonomous navigation. An accurate gravity gradient sensor is needed to alleviate this error. A highly sensitive gravity gradiometer has been demonstrated at the University of Maryland. (M. V. Moody, E. R. Canaan, and H. J. Paik, “Three-axis superconducting gravity gradiometer for sensitive gravity experiments,” Rev. Sci. Instrum., vol. 73, pp. 3957-3974, November 2002). The extremely high sensitivity of the device lies in the use of superconducting principles in the inductive sensing and on the general stability of material properties at extremely low temperatures. This “Superconducting Gravity Gradiometer (SGG)” employs linear accelerometers to independently measure the three diagonal, or in-line, components of the gravity gradient tensor.
The schematic circuit diagram for a gradiometer axis of the SGG is shown in FIG. 1. The instrument comprises a total of nine single-axis accelerometers. Six linear accelerometers are mounted on the axes of a precisely machined cube with the sensitive axes perpendicular to the faces of the cube. The test masses of the accelerometers on opposing faces are coupled by superconducting circuits to form three orthogonal gradiometers. In addition, three superconducting angular accelerometers (SAAs) are mounted with their sensitive axes aligned with the three gradiometer axes. In this arrangement, the differential linear accelerations correspond to the three diagonal components of the gravity gradient tensor. The three components of the platform linear acceleration are sensed as the common acceleration of the gradiometer test masses. These signals, along with the angular acceleration, permit correction of dynamic errors in all six degrees of freedom.
The test masses are levitated against gravity by storing a persistent current IL, using a resistive heat-switch RL, in the loop formed by LL1 and LL2 (in zero-g, a second circuit and current would provide an opposing force). The coils LS1 and LS2 are connected in parallel through a transformer to a SQUID to form a sensing circuit. The transformer Lt1 and Lt2 provides impedance matching and limits the dc current flowing through the SQUID input coils, LS3.
Though only one is shown, each axis of the gradiometer actually has two sensing circuits wound on the same coil form. In one circuit, the currents are stored in the same direction (using heat-switch RSS), as shown in FIG. 1. In this case, after obtaining the proper ratio of IS1 and IS2 to null sensitivity to common-mode acceleration, the SQUID detects the differential acceleration, or gravity gradient. In the other circuits, the currents are stored in the opposite direction (using RSP), and the SQUID detects the common-mode acceleration signal. Signal balancing is attained by means of stable persistent currents prior to detection.
Based on the technological success of the SGG, a variation that would be more suitable for the harsh motion environment of a land or sea vehicle is being assembled. This gradiometer makes use of angular accelerometers, very similar to those designed for the in-line SGG, to measure off-diagonal components of the gradient tensor (R. L. Forward, J. Appl. Phys. Vol. 50, pp. 1-6, January 1979; F. J. van Kann et al., Laboratory tests of a mobile superconducting gravity gradiometer,” vol. 165 & 166, pp. 93-94, August 1990). Due to its design, the cross-component SGG has a major advantage in obviating linear acceleration errors. This property is important on a land or sea vehicle since large linear displacements prohibit platform stabilization in the linear degrees of freedom, whereas rotational stabilization of instrument platforms is conventional.
A sensitive angular accelerometer has been developed (M. Vol Moody et al., “Principle and performance of a superconducting angular accelerometer,” Review of Scientific Instruments, March 2003, V. 74, Number 3, p.p. 1310-1318). Schematic of the angular accelerometer sensing circuit is presented in FIG. 2.
Three of these superconducting angular accelerometers (SAAs) have been constructed and tested in conjunction with the three-axis superconducting gravity gradiometer (SGG) with one angular accelerometer being used for each sensing axis of the SGG.
As in the design of the SGG, the Meissner effect and flux quantization are used to efficiently couple the test mass displacement to a SQUID amplifier.
In the SAA, a test mass, which is nearly semicircular in shape, is supported by a flexure pivot located at its center of mass (c.m.). Both the test mass and pivot are formed by a single cut made with a wire electric discharge machine (EDM) in a niobium (Nb) disk. The flexure is extremely rigid in all linear degrees of freedom and for rotations in the plane of the sensor leading to a rugged yet sensitive device. The cavities below the test mass provide pockets for the SQUID sensor, transformer, heat switches, and high current joints.
Elements of the displacement sensing circuit, schematically illustrated in FIG. 2, are housed in cavities machined into the disk. Spiral coils LS1 and LS2, for sensing test mass motion are located on the upper surfaces of these forms. The pockets in the lower half of the disk contain the heat switches (small carbon resistors for storing current), a Quantum Design dc SQUID, superconducting wire-to-wire joints, and an impedance-matching transformer. The Nb housing and cover plates form an optimum magnetic shield for the SQUID and other circuit elements. The size and circular shape permit ease of mounting to the SGG.
The sensing coils are connected in parallel to the SQUID input coil, LS3, through the impedance-matching transformer, Lt1 and Lt2. Two heat switches permit flux to be trapped in either the same (with heat-switch RSS) or opposite sense (with heat-switch RSP). Heat-switch RSQ is turned on while storing current to protect the SQUID. In the first case, persistent current initially flows only through the two sensing coils. In the second case the persistent current flows through the transformer primary and then splits between the two coils, which is the primary operating mode. In this case, an angular deflection of the superconducting test mass increases the inductance of one coil and decreases that of the other through the Meissner effect. This forces current to flow through the transformer, generating a signal at the SQUID proportional to test mass deflection.
Despite the satisfactory characteristics of the SGG employing an SAA at each gradiometer axis as presented in previous paragraphs, these SAAs were used to measure angular motion of the instrument platform but not gravity gradients. The SGG instrument presented in the paper of M. Vol Moody, et al, published in 2003, consists of six linear and three angular accelerometers. By measuring differences between pairs of linear accelerometers, the device provides the three diagonal components of the gravity gradient tensor. The angular accelerometers were used to determine errors in the SGG that couple to angular motion. This SGG was developed for space applications, where the acceleration environment is relatively mild. However, the platform motion environment of an aircraft, ship or land vehicle is far more hostile than that of a spacecraft. The angular motion can be controlled using a stabilized platform; however, the translational acceleration cannot be controlled as the translational displacements often exceed the dimensions of the vehicle. The linear accelerometers used in the prior SGG to measure the gravity gradients would suffer large displacements under the expected motion environment of an aircraft or ship. Under such large displacements, the nonlinear terms in the accelerometer transfer functions would mask any expected gravity gradient signal.
It is therefore desirable to have an instrument operating in extremely harsh environments to measure gravity gradients, which has high linearity in large acceleration environments, broad bandwidth, and reduced sensitivity to motions of the platform to which the instrument is mounted.
Further, although the cryogenic instrument benefits both from the reduced thermal noise and from the stability of superconducting circuits and material properties at near-zero temperatures which permits precise matching of scale factors and accurate rejection of dynamic errors, one of the main perceived drawbacks of the SGG however has been the necessity for a liquid helium cryostat.
Therefore, a SGG free of a liquid helium cryostat would be desirable.